1. Field of the Invention
The present invention relates to a mixer for mixing of plastifiable materials mainly such as rubber and plastics materials.
2. Description of the Related Art
In this type of mixer, as shown in FIG. 17, a pair of rotors 5 and 6 are rotatably mounted in parallel and in opposite directions within a mixing chamber enclosed with a casing 2 and end frames 3A and 3B; a material charging opening 9 is provided in the top of the mixing chamber 4 and a force ram 10 for applying a pressure to charged materials is removably mounted in the material charging opening 9; the force ram 10 being connected to the outer end of a piston rod 12 of a ram cylinder 11 whereby the force ram 10 is installed and removed and the pressure is applied to the charged materials.
On the shafts 5A and 6A of both rotors 5 and 6 are fixedly installed a connecting pinion 7 and a connecting gear 8 respectively. To the end of the shaft 6A of the connecting pinion 7 is connected a speed reducer 41, which is coupled to the output shaft of a main motor 42. To the ram cylinder 11, compressed air supply-exhaust tubes 15 and 16 are connected through ram-lift changeover solenoid valves 13 and 14. The solenoid valve 13 for the compressed air supply-exhaust tube 15 on the cylinder head side is designed to prevent overloading the motor 42 during mixing operation by cutting off a maximum load through de-exciting the solenoid valve 13 by the output of a contact-type ammeter 43 attached to the motor 42.
The rotors 5 and 6 are designed to be rotated in the illustrated directions by the pinion 7 and the gear 8 having no common measure. That is, the number of teeth on the pinion is not wholly divisible by the number of teeth on the other opinion. The phase of both rotors 5 and 6 changes correspondingly to the gear ratio (generally 1.1 to 1.2) of the pinion 7 and the gear 8, and the rotor power distribution chart, that is, the load current chart, of the motor 42 indicates as great a peak load as 130 to 200 percent during the initial period of pressure application, as shown in FIG. 21. As a result of tests, the range of material mixing by both rotors 5 and 6 is as shown in FIG. 18; it has been proved, however, that when the blade tips a and b of both rotors 5 and 6 face inwardly toward each other (phase difference 0 degree) as shown in FIG. 19(a), there occurs the maximum load (maximum peak current), and then a next greatest load (peak current) in the vicinity (see FIG. 19(b)) of this combination.
That is, in FIG. 19(b), when the phase difference of the rotor 5 is within the range of .theta.1 to .theta.2, the peak current occurs; and as is clear also from the chart in FIG. 21, the current value is 120 percent or less when both .theta.1 and .theta.2 are 45 to 90 degrees in relation to the 100-percent rated current of motor, 180 to 200 percent when .theta.1 and .theta.2 are 0 to 10 degrees, 140 to 150 percent when both .theta.1 and .theta.2 are 15 to 20 degrees, and 120 to 130 percent when both .theta.1 and .theta.2 are 25 to 45 degrees. Therefore, a conventional set current on the ammeter 43 is 120 percent of the rated current of the motor.
Pressure application by the force ram 10 to the material M charged into the chamber 4 starts as shown in FIG. 20(a), moving the force ram 10 downwardly with the compressed air supplied to the head side chamber 11A of the ram cylinder 11 through the solenoid valve 13 and the air supply-exhaust tube 15 to thereby apply the pressure to the material. The charge of the material is generally of the order of 70 percent of the volume of the chamber 4. Since the apparent specific gravity of the material is small, the force ram 10 stops halfway without going downwardly as low as the bottom dead center.
The material that has entered both rotors 5 and 6 is crushed and mixed, being gradually forced by the force ram 10 as the specific volume of the material M increases. There occurs a difference in mixing between a material previously supplied and a material subsequently supplied to both rotors 5 and 6, the difference becoming a cause of spotty quality. Some material M1 remains unmoving on the lower surface of the force ram 10 and in a space S (see FIG. 20(b)) outside of the working radius R (see FIG. 17) of both rotors 5 and 6.
According to a prior technique, the force ram 10 is automatically or manually moved upwardly at a specific material mixing time and temperature, to thereby move the material M1 staying in the space S, thus agitating, and giving cuts to, the material in order to prevent uneven mixing.
In the above-described prior technique, there has been such a disadvantage that an excessive load occurs to give damage to the rotor drive system when the phase angles .theta.1 and .theta.2 of both of the rotors 5 and 6 are 0 to 2 degrees. Since a peak overload occurs at approximately one-fourth a turn of both rotors 5 and 6 even if the pressure of the force ram 10 is released at the time of peak overload, no conventional means can quickly release the pressure, and immediately cope with the peak overload. There exists such a disadvantage, therefore, that the mixer is required to be provided with a powerful drive system, and that the mixing effect will be reduced.